1. Field of the Invention
The present invention relates generally to phase locked loop data decoders and more particularly to a decoding system for detecting digitally encoded data such as NRZI (non-return to zero input) type group code recorded data having varying degrees of bit shift. While the present decoding technique is not limited to any particular type of data encoding scheme nor to any particular data containing medium, the invention is described with reference to data recorded on magnetic tape.
2. Description of the Prior Art
Present day data processing, storage and retrieval systems frequently employ magnetic tape as a data storage medium, upon which data is stored in the form of magnetized areas, between which areas exist magnetic flux transitions. The position of a particular flux transition on the magnetic tape may be defined as that point which exhibits the maximum free space surface flux density normal to the tape surface. The space allocated for a single data bit on the magnetic recording medium is defined as the bit cell length, while the nominal distribution per unit length of cells on the magnetic recording medium is the data density. As the data density with which information is recorded on magnetic tape increases, various undesirable effects occur which affect the reliability of the recovery of such data. The magnetic interaction of the densely distributed magnetic flux transitions, known as "bit crowding", produces an effect called bit shift or peak shift, in which the transitions of the recorded data read from the magnetic medium are moved or shifted from their proper positions in time within the bit cells. For a particular density of recording, the bit shift will also substantially vary depending upon the method by which data is encoded on the magnetic medium. A well known encoding technique is the NRZI encoding technique, by which a flux reversal occurs whenever a logical "one" is recorded, with no fixed magnetization state being assigned to the "one" state. Thus, a flux reversal or transition is recorded for each logical "one" while no flux reversals or transitions occur for logical "zeros". When NRZI data is read back from a magnetic medium bit shift is especially pronounced whenever one or more logical zeros occur following (or preceding) a long string of consecutive logical ones in a random fashion. Another problem which occurs when a number of zeros greater than two must be decoded is phase drift and resultant loss of synchronization in a phase locked loop decoder, which remains synchronized to the incoming data only when transitions are continuously detected. This clock loss, which results from attempting to decode an indeterminate number of zeros is sometimes referred to as "downstream bit shift effect". Additionally, changes in the rate at which data is encoded or read, such as variation in the instantaneous tape speed in magnetic tape systems causes variation in the bit cell width.
Encoding techniques used in present magnetic recording reduce, but do not totally eliminate the above-described decoding problems. One such encoding technique is Group Code Recording (GCR), by which groups of characters or data bits are encoded prior to being recorded on the magnetic recording medium. In an exemplary GCR recording, a four-digit code is converted into a five-digit code which is configured in part such that no more than two data "zeros" occur serially. This coding is described in detail by U.S. Pat. No. 3,639,900 of H. C. Hinz, Jr. and U.S. Pat. No. 3,624,637 of J. W. Irwin. Such encoding techniques are sometimes called run-length-limited codes since the number of zeros which can occur in succession is limited. The Irwin patent also describes a resynchronization system in which special indicia are recorded and interleaved among the data signals.
Various self-synchronization systems have been proposed by which the readback data is clocked by means of a variable frequency oscillator. Such systems are referred to in U.S. Pat. No. 3,789,380 of M. R. Cannon, which patent also describes a digital recording system in which data is recorded and read back from a magnetic medium in association with a control signal having an integral multiple of the given repetitive frequency for clocking the data to provide readback synchronization. Cannon enhances resynchronization of recorded data by frequency interleaving resynchronization signals or marker points in the recorded data.